As a new type of solvent with immeasurable vapor pressure, room-temperature ionic liquids are being used for chemical separation and unique reaction media. Solvent phase behavior is an important factor in the attractiveness of using ionic liquids in these applications as well as in new applications such as absorption cooling or heating.
Vapor compression and absorption refrigeration cycles are well-known methods of cooling and are described by Haaf, S. and Henrici, H. in “Refrigeration Technology” (Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, Wiley-VCH Verlag GmbH, Weinheim, Germany, Volume 31, pages 269-312). The basic cooling cycle is the same for the absorption and vapor compression systems. Both systems use a low-temperature liquid refrigerant that absorbs heat from water, air or any medium to be cooled, and converts to a vapor phase (in the evaporator section). The refrigerant vapors are then compressed to a higher pressure (by a compressor or a generator), converted back into a liquid by rejecting heat to the external surroundings (in the condenser section), and then expanded to a low-pressure mixture of liquid and vapor (in the expander section) that goes back to the evaporator section and the cycle is repeated. The basic difference between the vapor compression system and absorption system is that a vapor compression system uses an electric motor for operating a compressor used for raising the pressure of refrigerant vapors, and an absorption system uses heat for compressing refrigerant vapors to a high-pressure.
Absorption chillers have been combined with vapor compression chillers in “hybrid” central plants to provide cooling at the lowest energy costs; for example the absorption chiller will be operated during high electric peak load when charges are high, whereas the vapor compression chiller will be operated during low electric peak load when charges are low, resulting in a more economical system. It would be desirable to have one system that integrates components of both the vapor compression and absorption cycles.
Vapor compression systems generally use ammonia or fluorocarbon derivatives as refrigerants, whereas absorption cycles generally use ammonia/water or lithium bromide/water. The two systems are not compatible in that fluorocarbon derivatives are not very soluble in water. While ammonia could be used for both systems, the toxicity and flammability associated with ammonia makes this option less desirable.
Although U.S. patent application Ser. No. 11/346,028, which is incorporated in its entirety as a part hereof for all purposes, discloses an absorption cycle wherein refrigerant pairs comprising at least one refrigerant and at least one ionic liquid are utilized, a need remains for systems to run a hybrid vapor compression—absorption cycle utilizing a refrigerant pair comprising at least one refrigerant and at least one ionic liquid.